The present invention relates to a method of condensing vapors, using an inert gas in its liquid phase as an expendable refrigerant by injecting the liquified gas directly into the gas stream containing the vapors to be condensed.
There are numerous industrial processes that emit condensable vapors or gases, the economical recovery of which would be desirable as the vapors represent a source of pollution as well as a valuable resource being lost. Such processes include, for example, the industrial drying of paints, inks and adhesives, as well as the sterilizing of sanitary components and certain foodstuffs in gas filled autoclaves, and even the storage of volatile liquids in large tanks which must be vented for safety.
In some cases a substantial portion of the vapors may be condensed and recovered by heat exchange with ordinary chilled water. Very often, however, the vapors are too diluted or the materials too volatile for condensation at the temperature of cooling water to be effective. In such cases, cooling water must be replaced or augmented by more intense refrigeration.
Mechanical refrigeration of exhaust gases has been used in the recovery of solvents from, for example, paint ovens. However, mechanical refrigeration systems for this purpose, scaled to industrial needs, are large and very expensive and energy intensive. The disadvantages of mechanical refrigeration systems are compounded where the materials to be recovered are extremely volatile. In practice, such systems are limited to condensation temperatures of about -20.degree. F. or higher. This may be adequate to condense a vapor such as toluene in some applications, for example, but not cold enough for substantial condensation of methyl-ethyl-ketone or ethylene oxide, which could require condensation temperatures of about -60.degree. F. and -140.degree. F. respectively.
U.S. Pat. No. 4,150,494 describes a system using liquid nitrogen refrigeration to condense and recover solvents vaporized in cure ovens. Liquid nitrogen, with a normal boiling point of about -320.degree. F., has ample refrigerating capability for recovery of most common solvents and process gases. However, it is so cold that its use as a refrigerant is difficult to control. Most solvents will freeze, rather than condense, upon contact with liquid nitrogen. The patent does not describe the manner in which heat exchange with liquid nitrogen is to be accomplished, but stipulates that it should be performed so as not to freeze the solvents.
The system described in U.S. Pat. No. 4,150,494 is now in commercial use. Its final stage of refrigeration, utilizing liquid nitrogen, is a packed column with a reservoir at its bottom for storage of liquid solvent, and a recirculation of chilled liquid solvent through the packing. Liquid nitrogen is poured directly into the solvent reservoir to cool it, and the device relies on the agitation of a recirculation pump and of boiling nitrogen to prevent freezing and maintain a controllable average temperature in the reservoir. The column is only marginally stable and is subject to the entire solvent reservoir freezing solid in the unpredictable industrial environment, in which case the entire system is unusable for several hours while the solvent melts. The column also needs to be primed with a large quantity of liquid solvent before it can be used, which makes it inconvenient or unsuitable in many cases. U.S. Pat. No. 4,133,663 described a system using liquid nitrogen for condensation of vinyl chloride by simple heat exchange. However, vinyl chloride has a very low freezing point, about -245.degree. F., not much above the boiling point of liquid nitrogen, particularly if the nitrogen is pressurized, and has therefore less tendency to freeze during momentary lapses of flow or control. The system is not suitable for condensing the numerous materials with much higher freezing points.
U.S. Pat. No. 4,122,684 describes a two-stage heat exchanger in which an intermediate fluid carries heat from the condensing vapor to the liquid nitrogen. The intermediate fluid is boiled by the vapor stream and condensed by the nitrogen, and circulates between the two. The system is relatively stable against freezing, but is expensive.
Nitrogen gas has also been used as an intermediate fluid, in which case liquid nitrogen is sprayed into the nitrogen gas to chill it and the chilled gas circulates through a heat exchanger to condense the vapor. The chilled nitrogen may be circulated by a blower or by natural convection. Only one heat exchanger is required, as compared with two heat exchangers where the intermediate fluid is isolated from the liquid nitrogen as well as from the condensing vapor, but still the equipment is large and expensive.
It is an object of the present invention to provide a method of utilizing a liquified gas such as nitrogen to condense process vapors without the size and high cost inherent in conventional heat exchangers.
It is a further object of the invention to provide a method of condensing vapors with liquid nitrogen which deters freezing of the vapors which are being condensed.
It is still a further object of the invention to provide a system for condensing vapors which is simple to construct and to operate, low in cost, and operable over a wide range of process conditions.